The dropwise dosing of liquids into a reactor plays a role in analytical technology, for example. Frequently, in such applications, analytes or reagents need to be dosed. The dropwise dosing of liquids should generally fulfill the purpose of introducing a defined volume of a substance into the reactor.
For example, an application important for waste water analysis, in the case of which the analyte is to be metered dropwise into a reactor, is the determining of carbon content and/or nitrogen content in wastewater, for example, the TOC (Total Organic Carbon, total organically bound carbon) or the TNb (Total Nitrogen, total bound nitrogen). In the case of known methods for determining these parameters, a liquid sample of small volume of, for example, some 100s of μl is fed dropwise to a reactor of a high temperature decomposition system. In the reactor, which, for example, is provided by a high temperature reactor formed as a pyrolysis tube, the organic ingredients are thermally decomposed to CO2 and the nitrogen containing ingredients to nitrogen oxide NOx. The acronym NOx stands here for a mixture of nitrogen oxides with nitrogen in different degrees of oxidation, which, however, has NO as the main component. In the decomposition in the high temperature reactor, there arises a gas mixture, which besides CO2 and NOx contains gaseous H2O and, in given cases, other pyrolysis- and reaction products of substances contained in the sample. The gas mixture is, with the assistance of a carrier gas (which, as a rule, also delivers the oxygen needed for the reaction) flowing permanently through the reactor, transported through a cooler having a water separator, a gas filter and an analytical unit. The amount of the occurring CO2, or NOx, is determined, for example, by infrared measurement or by chemiluminescent measurement, and, therefrom, the TOC—, or TNb content of the liquid sample determined.
The temperatures reigning in the high temperature decomposition system lie during operation significantly above the boiling point of the dosed liquid sample. In the case of TOC— or TNb determination, there rules in the interior of the reactor usually a temperature of about 650° C. up to 1300° C., depending on whether the decomposition of the sample is supported supplementally by a catalyst. In contact with the wall of the reactor or other surfaces present within the reactor, a liquid drop reaches, within a very short time, boiling temperature and, respectively, the reaction temperature required for the reaction with the oxygen contained in the carrier gas. A liquid drop dosed into the reactor transforms into the gas phase, consequently, directly after the dosing, by evaporation and/or by forming gaseous reaction products.
In determining the analyte concentration, for example, the TOC— or TNb-value, it is necessary to know the volume of the sample liquid metered into the reactor exactly. Especially, in the case of small sample volumes in the μl-, or lower ml-range, defective metering can lead to intolerable departures of the ascertained analyte concentration from the actually present analyte concentration. If one assumes, for example, that a volume of 400 μl of an aqueous solution corresponds, for instance, to 20 drops, then there results in the case of a defective dosing, in the case of which only 19 drops were metered into the reactor, already a deviation of the actually dosed sample volume from the predetermined sample volume of around 5%.
Defective metering can, however, exactly in automated operation of an analytical apparatus, occur again and again, for example, by plugging of the sample supply line by solid particles contained in the sample or due to gas bubbles in the sample to be metered. It is, consequently desirable, for assuring a correct analytical result, to monitor the metering of the sample into the reactor.
In infusion technology, for example, from EP 237773 A1, it is known to detect drops by optical means, for example, by light barriers, and, in this way, to monitor, how many drops of a liquid are actually dosed. Such an apparatus is, however, structurally complex, and especially as regards the described applications in the high temperature range only difficultly implemented. Even when the reactor of the high temperature decomposition system is transparent, for example, formed from quartz glass, and the light barrier is arranged outside of the pyrolysis tube, such a monitoring of the dosing of drops into the pyrolysis tube is not practical, since the transparency of the quartz glass decreases over the duration of operation due to the deposition of salts and through local crystallization of the quartz glass matrix through the influence of alkalai metal ions at high temperatures.